Field of the Invention
The present invention relates to image capturing apparatuses and control methods thereof.
Description of the Related Art
An image sensor in which a plurality of photodiodes are arranged two-dimensionally is used in a typical digital camera. Thermal noise produces dark current in the photodiodes in such an image sensor, and such dark current is a cause of a drop in dynamic range and deterioration of image quality.
Accordingly, a method is used in which a pixel that has a photodiode and a pixel that does not have a photodiode are disposed within a light-shielded pixel region (an optical black region), a dark current amount is estimated based on a difference between output signals from the stated pixels, and image capturing pixels are corrected. Meanwhile, because dark current depends on the temperature and increases as the temperature rises, a method that carries out correction based on the temperature has been proposed (Japanese Patent Laid-Open No. 2008-118293).
An analog gain based on an ISO sensitivity is multiplied within the image sensor in a typical image capturing apparatus. An output signal from the image sensor then undergoes analog-digital (A/D) conversion in a signal processing circuit, and dark current correction, color interpolation processing, and so on is carried out.
Incidentally, because dark current depends on the temperature, an accumulation time, and so on, dark current increases in high-temperature environments and during long exposures. Furthermore, in the case where an analog gain is multiplied within the image sensor, dark current is also gain-multiplied as a result, and thus the dark current amount increases at higher ISO sensitivities. There is thus the possibility that a dynamic range of the A/D conversion carried out in the signal processing circuit will be exceeded and saturation will occur.
In this case, the dynamic range necessary for image processing carried out by signal processing circuits in later stages cannot be ensured, and a correction method such as that described in Japanese Patent Laid-Open No. 2008-118293 will therefore no longer be usable. Note that the same problem occurs in configurations where A/D conversion is carried out in each of column readout circuits within the image sensor.